The present invention relates to optical communication systems, and more particularly to amplification in optical communication systems.
The enormous growth in Internet traffic is challenging the telecommunication industry to develop technology that will greatly expand the available bandwidth of backbone communication networks. Further improvements in optical communication technology hold great promise to meet continuing demands for greater and greater bandwidth.
Wavelength Division Multiplexing (WDM) technology, in particular dense WDM (DWDM) techniques, permit the concurrent transmission of multiple channels over a common optical fiber. The advent of Erbium-doped fiber amplifiers (EDFAs) has accelerated the development of WDM systems by providing a cost-effective optical amplifier that is transparent to data rate and format. An EDFA amplifies all the wavelengths simultaneously, enabling the composite optical signal to travel large distances (e.g., 600 km or greater) without regeneration.
One of the principal limitations of EDFA technology is limited bandwidth. Discrete and distributed Raman amplifiers have been developed to overcome this limitation. Raman amplifiers provide very high gain across a wide range of wavelengths, provide improved optical signal to noise ratio (OSNR) system performance, and also drastically reduce impairments due to fiber non-linearity.
Successful use of Raman amplification requires flat gain across the spectrum occupied by the WDM signal. Flat gain can be provided by the use of gain flattening filters (GFFs) but their utility is limited due to their insertion loss which degrades OSNR performance. As the gain variation that the GFF must compensate increases so does the insertion loss. The degradation of OSNR detrimentally reduces the span length that may be achieved without optical amplification and the link length that may be achieved without regeneration of the optical signal from recovered data.
One way of achieving flat gain in Raman amplification is by using wavelength multiplexed pump lasers. However, even using this technique, further increases in the desired bandwidth of the WDM signal to accommodate increases in data traffic cannot be achieved without the detrimental addition of a gain-flattening filter.
What is needed are systems and methods for optical amplification of WDM signals that provide flat gain across wavelength without sacrificing OSNR performance.